1. Field of the Invention
This invention relates to coronary stent deployment, and in particular, to improved image guidance in coronary stent deployment.
2. Description of the Related Art
Increasing numbers of percutaneous transluminal coronary angioplasty (PTCA) are being performed in the United States. However, after PTCA, restenosis of the dilated segment occurs in a large percentage of patients and results from elastic recoil, neointimal formation and vascular contraction. PTCA followed by coronary stent placement provides a luminal scaffolding that virtually eliminates recoil and remodeling, and has been shown to significantly reduce the likelihood of restenosis. Currently, a vast majority of patients receive stents after angioplasty. In such procedures, cardiologists frequently deploy multiple adjacent stents in an artery to treat extended lesions or dissections. Since it is important to accurately align the stent ends, the cardiologist must adjust the position of the catheter head relative to a previously deployed stent. This requires that the first stent and the catheter head be visualized well enough that their relative stent positions can be accurately determined. This has not been possible in conventional coronary stent deployment techniques. As a result, a subsequently placed stent often cannot be placed precisely in relation to a previously placed stent, resulting in either an overlap or a gap between the two stents. Gaps between stents are significant because of the risk of residual dissections and restenosis. Overlap of stents increases the risk of restenosis due to increased vessel injury during deployment. In addition, studies with intravascular ultrasounds (IVUS) imaging of deployed stents revealed that a high percentage of stents may be insufficiently dilated despite an apparently angiographically successful deployment.
Deployed stents may be evaluated using intravascular ultrasounds (IVUS) imaging technique (S. Nakamura et al., xe2x80x9cIntracoronary ultrasound observations during stent implantation,xe2x80x9d Circ., 89, pp. 2026-2034, 1994). IVUS can accurately define the anatomy of the vessel and the stent within the vessel and is considered the gold standard for defining the results of stent implantation. But such assessment is time-consuming and expensive, and is not useful for visualizing previously deployed stents during the placement of multiple stents because the ultrasound probe cannot be inserted along with the delivery balloon and stent. Moreover, the IVUS device is invasive and may increase patient risk. IVUS may also physically compromise the integrity of the deployment of certain stent types. An alternative method for evaluating stent expansion is coronary pressure measurement (C. E. Hanekamp et al., xe2x80x9cComparison of quantitative coronary angiography, intravascular ultrasound, and coronary pressure measurement to assess optimum stent deployment,xe2x80x9d Circ., 99(8), pp. 1015-1021, 1999). Other alternative methods use medical imaging devices to evaluate stent implantation. One reference describes a videodensitometric analysis, in which density profiles are constructed and compared with a theoretic profile of a normal artery (Y. Rozenman et al., xe2x80x9cQuantitative videodensitometric technique for verification of optimal coronary stent implantation,xe2x80x9d International Journal of Medical Informatics, 51(1), pp. 51-57, 1998). However, such measurements are typically imprecise, as evidenced in the Rozenman study by the weak correlation of only R=0.74 between the videodensitometric and IVUS measurements of stenosis.
Most coronary stents are insufficiently radiopaque and are difficult to visualize in x-ray angiograms. Thus, in evaluation techniques using medical imaging devices, it is desirable to increase the visibility of the stents. For example, the stents may be coated with radiopaque material such as gold to increase their visibility. Experimental studies with gold plated stents show results with less thrombogeneity of the stents, but clinical comparison of stainless steel stents and gold-plated stents show significantly increased restenosis rates with gold plating (A. Schomig et al., xe2x80x9cRandomized comparison of gold-plated steel stent with conventional steel stent: Results of the angiographic follow-up,xe2x80x9d Journal of the American College of Cardiology, 33(2), pp. 95A, 1999). Stents and guidewires having radiopaque markers attached thereto have been described in numerous references, examples including Frantzen, U.S. Pat. No. 5,741,327 (stent with markers) and Lorenzo, U.S. Pat. No. 5,836,892 (guidewires with markers). In techniques using medical imaging devices, images are taken to observe the stents, but these images are typically still images that show only the final positions of the stents or guidewires or balloons after placement.
Various image processing techniques have been proposed and are generally applicable to enhance angiographic images. For example, background subtraction attempts to separate the coronary arteries from patient background structures. But background structures can cause tracking errors or densitometric errors in correctly tracked arteries, since background structures are superposed on the vessel in the images. In the absence of motion, digital subtraction of a mask image taken before the contrast injection may be performed to remove the background structures, but this method has not been clinically successful because of involuntary patient motion. Motion-correction in current clinical DSA (digital subtraction angiography) systems is accomplished by a manually controlled translation of the mask image with respect to the contrast image. Unfortunately, since cardiac motion is more complex then simple translations, this technique will reduce artifacts in some parts of the image but reinforce or create artifacts in other parts of the image. Other methods to improve accuracy have been proposed, including a system for automatic re-masking during image acquisition, in which a new mask is selected whenever a similarity measure drops below a certain threshold, and a system using control points to determine the motion between the mask and live images. Manual selection of control points, however, can introduce errors if the points are not corresponding between the pre-processed and the mask images. Another technique, known as flexible mask subtraction, automatically tracks features from the mask to the live image. This method requires prior segmentation of the vessel from the surrounding background; as a result, regions beneath the vessel are not tracked directly but are interpolated. In general, a 2 D warped mask subtraction method involves subtraction of a previously acquired image that has been warped to correct for motion (E. H. W. Meijering et al., xe2x80x9cRetrospective motion correction in digital subtraction angiography: A review,xe2x80x9d IEEE Trans. Med. Imag., vol. 18, No. 1, pp. 2-21, 1999).
All of the above-mentioned background subtraction techniques suffer from several important limitations. First, they use a xe2x80x9cmaskxe2x80x9d image of the background taken prior to stent deployment. The long delay between acquisition of the mask image and the image with stents present makes involuntary patient motion (such as breathing) a major source of degradation. Second, the subtraction of two images containing random noise results in an image with more noise than the original images. Third, the three-dimensional background motion is modeled as a single two dimensional motion. Only the portion of the background which moves according to the estimated two-dimensional motion mapping is correctly subtracted.
One method for reducing random noise in moving images is motion-compensated temporal averaging (e.g. Dubois E, Sabri S, xe2x80x9cNoise Reduction in Image Sequences Using Motion-Compensated Temporal Filteringxe2x80x9d, IEEE Trans. Comm. 32(7):826-831, 1984). A feature such as a stent can be tracked and shifted to a common position in each image prior to temporal averaging. However, in projection images such as angiograms the presence of background structures can cause tracking errors which blur the time-averaged image.
The method of moving layer decomposition was developed in order to improve the accuracy of quantitative measurements made from coronary angiograms (QCA). This technique performs tracking and motion-compensated temporal averaging of different image structures (layers) to achieve both background removal and noise reduction. However, in fluoroscopic images of deployed stents the stent signal is usually too weak to track accurately. In angiographic image sequences containing opacified arteries, tracking errors can result from the overlapping vessel and background structures.
In summary, prior methods for enhancement of angiographic images are inadequate to assist stent deployment. Background subtraction techniques increase random noise which degrades stent visibility and quantitative measurements of lumen shape. Tracking techniques suffer from errors due to the presence of background structures and the faintness of the stent in the images.
In view of the prevalent use of stenting in coronary interventions, the adequacy of angiographic guidance and evaluation of stent deployment needs to be improved.
Accordingly, the present invention is directed to a method for improving coronary stent deployment by applying a multiframe analysis that substantially diminishes one or more of the problems due to limitations and disadvantages of the related art.
An object of the present invention is to provide a method to improve clinical visualization of stents during and following deployment.
Additional features and advantages of the invention will be set forth in the descriptions that follow and in part will be apparent from the description, or may be learned by practice of the invention. The objectives and other advantages of the invention will be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.
According to the present invention, radiopaque markers on the delivery balloon, guidewire, or other device are used to provide a trackable feature that is co-moving with the stented vessel. Placement of a marked device into a vessel lumen enables one to enhance the visibility not only of the marked device, but also of any previously deployed stents nearby in the same vessel. A cine x-ray (fluoroscopic) image sequence is acquired while the radiopaque markers are inside the stented lumen. Enhanced images of the stents are produced by layer decomposition of the fluoroscopic image sequence. This decomposition into moving layers is performed by tracking image features and performing motion-compensated temporal averaging of background and stent features.
In another aspect, the present invention provides a method for accurate deployment of coronary stents with no overlap or gap between multiple stents. Accurate positioning of an additional stent is obtained by repeated adjustment and assessment of the relative positions of the deployed and undeployed stents using enhanced fluoroscopic images obtained by the procedure described above.
In another aspect, the present invention provides a method to improve assessment of stent dilation. In this case the vessel is filled with a contrast agent during imaging. Layer decomposition is performed in order to yield background-subtracted and time-averaged images of the layer containing the opacified vessel lumen. In order to improve tracking accuracy, the time-average of the image sequence (stationary background layer) is subtracted prior to tracking the vessel. In addition, the stented region is outlined and isolated from the rest of the image for use as a kernel for vessel tracking. The resulting vessel layer image can then be used to assess adequacy of stent expansion by visual or quantitative assessment of the relative amount of lumen narrowing or broadening in the stented region (e.g. percent diameter stenosis, densitometric area stenosis, or volumetric density deficit index described by Rozenman). If the stent is assessed to be insufficiently dilated, additional balloon inflations can be performed.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.